This invention relates to an apparatus and a novel thermally responsive control valve associated therewith capable of mixing the vapor of a volatile liquid anesthetic with a gaseous fluid, such as air, oxygen, or nitrous oxide or any other gas or a combination thereof, all of which for convenience are hereinafter included in the term "gas". More specifically, the invention relates to a bypass type of anesthetic vaporizer where the gas input to the vaporizer is divided into two streams which subsequently recombine. In addition, a control valve means is disclosed which includes a rotary dial which operates in a linear manner with respect to changes in anesthetic concentration resulting therefrom.
Most anesthetic vaporizers in use today are designed with a vaporizing chamber and bypass chamber arrangement. Within an inlet portion to such vaporizers the gas flow becomes divided into two separate portions. One portion passes through the vaporizing chamber where it is enriched or saturated with vapor of the liquid anesthetic agent while the other portion completely bypasses such vaporizing chamber. Finally, both of the separate gas flows recombine downstream from the vaporizing chamber and pass through an outlet portion of the vaporizer apparatus. The ratio of the two partial gas flows depends upon the ratio of resistances in the two respective paths or passages, that is the flow resistance in the bypass and the flow resistance in the vaporizing chamber.
The ratio of the resistance in the two gas flow passages can be controlled with the aid of a built-in throttle valve. Changes in the orifice of the valve alter the distribution of the main gas inflow, which in turn, alters the concentration of volatile anesthetic in the gas leaving the vaporizer.
A number of problems have been encountered in the design of anesthetic vaporizers which in large part are related to the safety requirements of such devices. In general it is most desirable for this reason that a user of an anesthetic vaporizer be able to specify a particular output flow having a particular anesthetic concentration under a variety of operating conditions and any factor detracting from this type of performance may be viewed as a problem associated with any particular vaporizer structure.
One such potential problem results if the portion of gas passing through the vaporizing chamber does not become fully saturated. The resultant effect is that the output concentration decreases as flow through the unit increases and accordingly such concentration could be at variance with that indicated by the settings on the apparatus. In addition, a most important feature of a vaporizer is that its output performance should be independent of ambient temperature, such temperature being reflected in the temperature in the input gas to the unit. This aspect of vaporizer design involves the fact that varying amounts of anesthetic vapor may be absorbed by input gases at different temperatures, generally input gases at higher ambient temperatures being capable of absorbing more anesthetic vapor. The simple solution to attaining an output performance which is independent of ambient temperature would be to keep the vaporizer at a constant temperature in an attempt to maintain gas flow through the unit at a controlled temperature to thereby control vaporization of an anesthetic in a knowledgeable manner. However, such a method would incorporate the additional problems of a temperature control system for the vaporizer and the potential malfunction of the component parts thereof which would in turn affect the performance of the vaporizer.
Another problem presented with the design of an accurate vaporizer apparatus concerns the heat loss in the liquid anesthetic due to the heat needed for vaporization thereof which is taken from the liquid anesthetic itself in the vaporizing chamber which results in a decresase in temperature therein and in a corresponding decrease in vapor pressure. This results potentially in a decrease in the delivered concentration of the vaporizer flow. Accordingly, it is most desirable that the alteration of the gas flow through the vaporizing chamber be readily adjusted in an automatic manner since temperature decreases resulting from vaporization can be quite rapid.